The present invention generally relates to vapor-compression cycle equipment, and more particularly to determining the level of refrigerant charge using low-cost non-invasive measurements obtained while the system is operating.
Vapor-compression cycle systems include air conditioners, heat pumps, chillers, refrigerators, coolers, etc. Proper refrigerant charge (the amount of refrigerant contained in the system) is essential for a vapor-compression cycle system to operate efficiently and safely. Charging charts are often employed to adjust an existing refrigerant level during the operation of vapor-compression cycle systems with refrigerant recovery. However, this technique does not provide quantitative information on charge level, and therefore can lead to a system being overcharged or undercharged. Current common practices for accurately determining the charge level in a vapor-compression cycle system require evacuating the system and weighing the removed refrigerant, a very time-consuming and costly procedure that involves removing existing mineral oil, recovering existing refrigerant, evacuating the system using a deep vacuum, and refilling the system with proper amounts of mineral oil and refrigerant.
In view of the above, various equipment and techniques have been proposed for diagnosing refrigerant charge levels in vapor-compression cycle systems. While most have been adapted to qualitatively indicate whether refrigerant charge is below or above acceptable limits, U.S. Pat. No. 6,571,566 to Temple et al. proposes a method for quantitatively determining system charge level. Temple et al. disclose that a quantitative determination can be obtained by establishing a relationship between at least one system operating parameter and refrigerant charge level, independent of ambient temperature conditions. For this purpose, Temple et al. disclose operating the system at various known refrigerant charge levels and under various known ambient temperature conditions, while monitoring the system with temperature sensors and pressure sensors to establish baseline data that can be used in an algorithm to determine refrigerant charge level during subsequent operation of the system. Temple et al. teach that, by measuring system pressures and temperatures while operating the system for a range of different refrigerant charges and ambient conditions, a model can be produced correlating the subcooling and superheat values of the system to corresponding refrigerant pressures. The model can be subsequently used to quantitatively determine the system charge level using empirical data regression.
Drawbacks to such an approach include the requirement to operate the system over a range of different refrigerant charges and ambient conditions, necessitating a considerable amount of labor to alter the ambient conditions and adjust the refrigerant charge, the latter of which incurs the risk of refrigerant leakage. Furthermore, pressure sensors are relatively expensive and their installation requires fittings that can further increase the probability of refrigerant leakage. The algorithm proposed by Temple et al. also is not well suited to monitor refrigerant charge level if faults other than incorrect refrigerant charge are present.
In view of the above, it would be desirable if an improved technique were available for non-invasively determining the refrigerant charge level in an operating vapor-compression cycle system.